The pair go head-to-head in the latest installment of the comic book movie franchise as superhero Tony Stark and his nemesis Mandarin, but they became instant pals while filming thanks to the Attenborough connection.
Downey, Jr. was up for an Oscar for his portrayal of Charlie Chaplin in Attenborough's 1992 movie Chaplin, while Kingsley walked away with the Best Actor Academy Award for historical epic Gandhi 10 years earlier.
The actors ensured the Jurassic Park star knew about their budding friendship by sending him a series of snaps from their time on set.
Downey, Jr. says, "We hadn’t worked together so we shared only Lord Attenborough, and yeah, of course, there was a connection.
"We took some pictures together to send DICkie (Attenborough), which must be the strangest visual for him to see - the two of us playing the bad guy and the good guy. We shot them out in front of a mansion in Miami, it was a magical place, actually."